1. Field of the Invention
The present invention relates to an endoscope illumination optical system.
2. Description of the Prior Art
Conventionally, endoscope illumination optical systems have been constituted by a light guide (surface light source) and a planoconcave lens element provided at an exit end of the light guide.
However, the planoconcave lens element cannot evenly distribute the light transmitted from the light guide, and the amount of illumination is not sufficient.
Endoscope illumination optical systems which include two positive lens elements are disclosed in, e.g., Japanese Unexamined Patent Publication (JUPP) No. S56-20428, JUPP No. H08-320440, JUPP No. 2000-275547, JUPP No. 2005-345787, and Japanese Unexamined Utility Model Publication No. H02-140519.
However, similar to the planoconcave lens element, the two positive lens elements cannot evenly distribute the light. Specifically, the periphery of a surface to be illuminated cannot suitably be illuminated. Namely, the light-utilization efficiency of an endoscope illumination optical system is insufficient.
Furthermore, JUPP No. 2002-182126 discloses an illumination optical system employing three positive lens elements; and JUPP No. H05-119272 and JUPP No. H05-157967 disclose an illumination optical system employing an aspherical surface. However, the production costs of these illumination optical systems are high.